1. Field of the Invention
The present invention relates to an X-ray mirror and an X-ray exposure apparatus which employs the X-ray mirror.
2. Description of the Related Art
Grazing incidence X-ray mirrors utilize total reflection on the surface of a subject, and thus must have a very high accuracy in shape and flatness. In recent years, X-ray sources having a high luminance, such as synchrotron radiation, have been employed. Thus, there is a demand for a grazing incidence X-ray mirror which exhibits high heat resistance and radiation resistance in addition to the above-mentioned characteristics. Generally, a grazing incidence X-ray mirror is obtained by optically polishing the surface of a substrate made of quartz obtained by coating silicon carbide on a base material, silicon carbide or carbon by chemical vapor deposition (CVD). In order to improve heat resistance and polishing characteristics, a substrate made of metal or silicon may be used. Further, in order to improve heat resistance, radiation resistance and the wavelength dependency of the reflectivity of the grazing incidence X-ray mirror, a metal layer made of, for example, gold or platinum, may be coated on the surface of the substrate made of any of the above-mentioned materials.
When the X-ray mirror is placed in a beam line of X-rays, such as synchrotron radiation and is illuminated with X-rays, a contaminating carbon layer is generated on the X-ray irradiating surface thereof due to residual organic substances present in the atmosphere. The attached contaminating carbon layer greatly varies the intensity and spectrum of X-rays reflected by the X-ray mirror. Hence, intensity correction must be conducted frequently or the mirror must be cleaned often. Further, variations in the position at which X-rays enter the mirror generate a distribution of the intensity of the reflected X-rays caused by the presence or absence of the contaminating carbon layer.
In recent years, the luminance of light sources have increased, and consequently, an influence of the contaminating carbon layer on the intensity and spectrum of the illuminating X-rays has thus become a serious problem.
Hence, attempts have been made to reduce the speed at which the contaminating carbon layer is attached by improving the degree of vacuum of the beam line or by cooling the mirror.
However, it has been reported that carbon contamination occurs even in a vacuum of 10.sup.-9 Torr. Further, the cooling method has been proposed to mainly improve the heat load of associated optical elements. Regarding the carbon contamination of optical elements, it has been reported that a reduction in the temperature of the optical elements tends to advance carbon contamination.
Further, a method of removing a contaminating carbon layer of the optical elements in a beam line has been proposed. This method facilitates cleaning of the mirror. However, when the mirror is significantly contaminated, it must be cleaned frequently. Thus, it is desired to restrict changes in the intensity of the reflected X-rays and reduce the number of times the mirror is cleaned.
In X-ray lithography, since the amount of X-rays to which a resist is exposed must be very accurate, the amount must be corrected even when the mirror is slightly contaminated. However, frequently conducted correction of the amount of X-rays exposed and mirror cleaning reduce throughput. Accordingly, it is desired to stabilize the intensity of X-rays reflected by the X-ray mirror over a long period of time. Further, in X-ray lithography employing synchrotron radiation as the light source, variations in the position of a beam vary the position on the mirror on which X-rays are illuminated, thus generating a distribution of the thickness of the contaminating carbon layer and hence irregularities of the X-ray reflectivity. This results in changes in the intensity of X-rays in the exposed plane and thus must be eliminated.
Japanese Patent Laid-Open No. Hei 4-158298 discloses a mirror in which a carbon layer having a thickness of 2 to 20 .mu.m is provided on the surface of a silicon carbide substrate in order to increase the coefficient of heat conductivity of the mirror. However, if the carbon layer having such a thickness is provided in order to achieve a reduction of changes in the reflectivity caused by the presence of a contaminating carbon layer, it is difficult to obtain a desired stability in reflectivity, because irregularities in the reflectivity caused by a distribution of the thickness of the carbon layer increase.